AC power flow is difficult and expensive to achieve. There are a variety of conventional AC power flow control solutions. Most modern power flow controllers are power electronic based. Examples of conventional power electronic based AC power flow control solutions include Back-To-Back (BTB) converters, unified power flow controllers (UPFCs), and controllable network transformers (CNTs). The basic principle for these types of devices is to use fast switching (active) converters to inject a variable voltage between sending and receiving ends of a line and hence control the power flowing through the line. Existing power flow solutions are implemented in the middle (not necessarily at mid-point) of a power line. A power flow controller in the middle of a power line controls how much power flows through the line. Alternate routes passively pick up the rest of power to be transmitted. The use of power electronic converters to partially or fully process the power flowing through a line makes these solutions complex and expensive. Other solutions such as thyristor switched series capacitors (TSSCs), phase shifting transformers (PSTs), variable frequency transformers (VFTs) and so-called smart wires (SW) partly or fully eliminate the need for power electronic converters. However, these solutions introduce unwanted system dynamics (TCSC), or generate complex fault modes (PST), or have high maintenance cost (VFT), or add line reactance (SW).
In addition, most conventional implement power flow control for all possible situations. As a result, these solutions are complex and expensive. The problem of power flow control can be segmented in various possible ways. In all situations, three or more power lines are connected at a junction or bus.
How quickly or often a power flow controller should work is one way of segmenting the power flow control problem. Typical power electronics based controllers allow continuous control, although exceptions such as VFT exist. Typically, power flow controllers which offer continuous control are much more expensive. The most prevalent reason for requiring a power flow controller is to prevent line overloading. As long as the line current is less than the line limit, this requirement is met. Hence in principle, a power flow controller should not be needed to control the line current over a continuous range. Thus, only a small subset of applications requires continuous control. Controllers such as phase shifting transformers and TSSCs implement discrete control. Discrete controllers are relatively less expensive and less sophisticated.